Field of the Invention
The present invention relates to a printed circuit board that is provided in an electronic apparatus and is used for data transmission according to a differential signaling system.
Description of the Related Art
In an electronic apparatus typified by a copier and a digital camera, data transmission rate has been enhanced in the electronic apparatus to achieve printing speed enhancement and high resolution image quality. To support the enhancement, data transmission rate is required to be improved. To achieve the improvement, it is necessary to increase the signal transmission frequency and increase the switching speeds of devices used for signal transmission. As a result, it is difficult for a conventional single-ended signaling to keep radiation noise below or at a limit value. Thus, changeover to a differential signaling system has been made.
The differential signaling system allows currents to flow in a pair of differential signaling lines in opposite phase. Accordingly, magnetic fields cancel each other. The cancellation is advantageous for reduction in radiation noise. Thus, the system has been widespread. However, in actuality, even the differential signaling system cannot eliminate radiation noise.
In general, in the differential signaling system, even if the electric properties of the pair of the differential signaling lines are designed to have the same configuration, radiation noise occurs. It is known that the cause is current components of differential signals with the same phase occurring in the inside of the semiconductor element that transmits the differential signals.
The current components in the same phase (in-phase current), i.e., common mode currents, are caused by a switching time lag between differential signals in a semiconductor element that transmits the differential signals, and a difference in rising and dropping characteristics of signal waveforms. The common mode currents are a main factor of causing radiation noise specific to the differential signaling system.
Current components with opposite phases (opposite phase current), i.e., differential mode currents, cancel each other's magnetic fields occurring therein. The cancellation exerts an advantageous effect of suppressing radiation noise. In contrast, common mode currents intensify each other's magnetic fields occurring therein. Accordingly, the intensification functions to increase radiation noise.
If a route for common mode currents returning to a semiconductor element for transmitting differential signals, which is a generation source thereof, is not secured, the current strays in various conductors residing in the electronic apparatus and returns to the semiconductor element through a stray capacitance of a printed wiring board. The electromagnetic fields caused by the straying common mode currents are measured as large radiation noise.
A conventional filter circuit described in Japanese Patent Application Laid-Open No. H04-372213 can rapidly return the common mode currents to the semiconductor element, and suppress radiation noise. FIG. 15 schematically illustrates a filter circuit proposed in Japanese Patent Application Laid-Open No. H04-372213.
In FIG. 15, a transmission semiconductor element 1 and a reception semiconductor element 21 transmit signals through wirings (differential signal wirings) 5 and 6 that transmit differential signals. On the differential signal wirings 5 and 6, a filter circuit 10 is provided in proximity to the transmission semiconductor element 1. The filter circuit 10 is connected to the reception semiconductor element 21 by cables 18 and 19, which are portions of the differential signal wirings 5 and 6. The filter circuit 10 includes: a capacitor element 7 arranged in electrical connection between a ground and the wiring 5 connected to a first signal output terminal 2 of the semiconductor element 1; and a capacitor element 8 arranged in electrical connection between the ground and the wiring 6 connected to a second signal output terminal 3 of the semiconductor element 1. A ground terminal 4 of the semiconductor element 1 is connected to the ground. The paths of common mode currents indicated by arrows in FIG. 15 pass through the wirings 5 and 6 and the capacitor elements 7 and 8 and return to the ground terminal 4. Such a configuration suppresses radiation noise from the cables 18 and 19, which are differential signaling lines.
In FIG. 15, the filter circuit 10 further includes inductor elements 13 and 14 and a capacitor element 15 to attenuate harmonic components of differential mode currents, thereby maintaining signal quality. A terminating resistor 20 is provided on output sides of the cables 18 and 19. In the filter circuit 10 illustrated in FIG. 15, chip-shaped capacitor elements excellent in high frequency property can be adopted as the capacitor elements 7 and 8.
A typical capacitor element used in the filter-circuit illustrated in FIG. 15 has an inductance due to a parasitic inductance caused by an electrode structure. The inductance due to the parasitic inductance is a significantly small value, which is several nanohenries. Accordingly, the inductance has caused no problem.
However, as the driving frequency for an electronic apparatus has been increased in recent years, a band causing a problem of radiation noise has expanded to a high frequency band. Accordingly, even a slight inductance of a capacitor element causes a problem of degrading the filter characteristics of a filter circuit. Furthermore, a high density packaging of a printed circuit board is also a factor of enhancing degradation in the filter characteristics of the filter circuit.
Thus, the present invention has an object to provide a printed circuit board that reduces the effective inductances of first and second capacitor elements with respect to common mode currents flowing in first and second capacitor elements to suppress radiation noise.